Comparing the demography of a forest herb, Trillium grandiflorum, in hedgerow corridors and forest patches

Conservation beyond the boundary of protected areas has become of major interest for biodiversity management. In agricultural landscapes, hedgerows have often been proposed as habitat corridors that might reduce the impact of habitat loss and fragmentation. Although presence of forest herbs has frequently been reported in these linear habitats, little is known about the processes that drive their dynamics. For three years, we monitored six populations of Trillium grandiflorum (Michx.) Salisb., a self-incompatible forest herb, to compare the demographic responses observed in hedgerows and adjacent forests using transition matrix models. In each population, we tested for pollen limitation and compared its magnitude across habitats, years, and sites. Additionally, we monitored plants’ reproductive output in response to isolation from forest patches and conspecific flowers in transplanted populations. Overall, no significant difference was detected among population growth rates (λ) computed in hedgerows and forests. In hedgerow, LTRE showed the negative contribution of lower survival rates in early development stages, this effect, however, being offset by higher growth rates. Contrarily to forests, where λ sensitivity is evenly distributed across development stages, prospective analyses showed that λ computed in hedgerows are most sensitive to survival of large reproductive plants. Although we observed pollen limitation in most populations, its extent did not differ between hedgerow and forest. Finally, seed set per fruit tended to decline as isolation increased. These results suggest that hedgerows can support viable T. grandiflorum populations. However, they also stress populations’ vulnerability to perturbations affecting the survival and spatial distribution of large reproductive plants in hedgerow.